How to constrain resizing in blender?
To minimize the scaling process ( or any transformation ) hold Shift after pressing S or after holding the manipulator handle.

When you are trying to make the object smaller make sure to place your mouse cursor as far as possible to have a space to move it inward ( and vice versa ), this may be problem with manipulator as it is relatively small and don't leave much room for movement inward for this you can try resizing the manipulator from User_Preferences➩Interface➩Manipulator➩Size :